var focus = document.querySelector('.focus');
var focusimg = document.querySelector('.focus').getElementsByTagName('img');
console.log(focusimg);
var promonavs = document.querySelector('.promo-nav').getElementsByTagName('li');
// 排他思想
for (var i = 0; i < promonavs.length; i++) {
    promonavs[i].onclick = function () {
        for (var i = 0; i < promonavs.length; i++) {
            promonavs[i].style.backgroundColor = '';
        }
        this.style.backgroundColor = '#ff5000';
        focus.style.backgroundImage = "";
    }

}